In any continuously variable transmission there is a device, referred to herein as a “variator”, which provides a continuously variable drive ratio. The variator couples to other parts of the transmission—typically gearing leading on one side of the variator to the engine and on the other side to driven components such as the driven wheels of a motor vehicle—through rotary input and output members. The speed of the output member divided by the speed of the input member is the “variator drive ratio”.
The concept of “torque control” is known in this art but will now be explained. It is useful to distinguish from the alternative of “ratio control”.
A ratio-controlled variator receives a control signal representing a required variator drive ratio. The variator responds by adjusting its drive ratio to the required value. The adjustment typically involves detecting the position of a ratio-determining element of the variator (e.g. the separation of the sheaves in a belt-and-sheave variator, or the position of the rollers in a toroidal-race type variator) and adjusting the actual position of this element to a desired position (determined by the control signal) using a feedback loop. Thus in a ratio controlled variator, ratio corresponds directly to the control signal.
This is not the case in a torque-controlled variator. Instead a torque-controlled variator is constructed and arranged such as to exert upon its input and output members torques which, for a given variator drive ratio, correspond directly to the variator's primary control signal. It is torque which is the control variable rather than drive ratio. Changes in speed of the variator input and output, and hence changes in variator drive ratio, result from the application of these torques, added to externally applied torques (e.g. from engine and wheels), to the inertias coupled to the variator input and output. The variator drive ratio is permitted to change correspondingly.
Torque control has to date principally been applied to toroidal-race, rolling-traction type variators. In an arrangement described for example in Torotrak (Development) Ltd's European patent EP444086, variator rollers serve to transmit drive between co-axially mounted input and output discs. The variator rollers exert respective torques Tin and Tout upon the input and output discs. Correspondingly the rollers experience a “reaction torque” Tin+Tout about the disc axis. This reaction torque is opposed by an equal and opposite torque applied to the rollers about the axis by a set of actuators. The geometry is such that movement of the rollers about the disc axis is accompanied by “precession” of tie rollers—a change in the angles of the roller axes to the disc axis, effecting a corresponding change in variator drive ratio. By controlling the actuator torque, the reaction torque Tin+Tout is directly controlled. The control signal in this type of variator corresponds directly to the reaction torque.
The actual torques exerted by the variator upon its input and output depend not only on the control signal but also upon the current drive ratio, since although the sum Tin+Tout is uniquely determined by the control signal, the magnitude of the ratio Tin/Tout. is equal to the reciprocal of the variator drive ratio, and so subject to change with the variator drive ratio. Nonetheless it can be appreciated that, for a given drive ratio, both Tin and Tout are uniquely determined by the control signal.
The direct correspondence between reaction torque and control signal is not provided by all torque-controlled variators. An example of a torque-controlled variator of a quite different type, using belt-and-sheave construction, is provided in the applicant's own prior European patent 736153 and its U.S. Pat. No. 5,766,105, wherein one sheave is mounted upon its drive shaft in such a way that motion of the sheave relative to the shaft along a helical path is permitted. Hence when torque is applied to the sheave, a corresponding force along the axis of the shaft is created. This axial force is opposed by a force applied to tie sheave by an actuator. Again, an equilibrium is created between the two forces. It can again be said of this example that the torque Tin exerted by the sheaves upon the shaft is, for a given variator drive ratio, uniquely determined by the control signal, which corresponds to the force applied by the actuator.
A feature common to both arrangements is that the variator comprises a component—the movable sheave or variator roller—whose position corresponds to the current variator drive ratio and that this component is subject to a biasing torque (or force) which is determined by the control signal and is balanced by the torques created at the variator input/output.
Effective utilization of torque-controlled transmissions depends on electronics to regulate the engine and transmission in unison. Early papers on the electronic control of such a powertrain are by Stubbs—“The Development of a Perbury Traction Transmission for Motor Car Applications”, ASME (The American Society of Mechanical Engineers) paper no. 80-GT-22, March 1980 and also by Ironside and Stubbs—“Microcomputer Control of an Automotive Perbury Transmission”, IMechE paper no. C200/81, 1981. Both papers describe a project concerned with electronic control of a transmission based on a toroidal-race rolling-fraction type variator operating in torque controlled mode.
Both papers point out an important advantage associated with continuously variable transmissions: that fuel economy can be greatly enhanced when using such transmissions by operating the engine at or close to the levels of engine speed and engine torque at which it is most fuel efficient. For any given level of engine power demanded by the driver there is a particular combination of engine speed and engine torque which provides best fuel efficiency. Stubbs plotted the locus of such “optimal efficiency” points on a graph forming a line representing the optimal engine efficiency. The control strategies proposed by Ironside and Stubbs were based on operating the engine on this line where possible.
In the control schemes described in these papers the driver's demand was interpreted as a requirement for wheel torque, which was then converted into a requirement for engine power by multiplication by the rotational speed of the vehicle wheels. From this power a unique point on the optimal efficiency line was selected, providing target values for the engine torque and engine speed. The engine was set to produce the target torque and the loading applied to the engine by the variator adjusted to bring the engine speed to the target value, using a closed loop based on engine speed.
Stubbs' simple approach proves inadequate, for a production motor vehicle, in a number of ways relating to stability of the transmission ratio and to driveability of the vehicle.
The challenges involved in controlling a torque-controlled transmission are very different from those involved in controlling a ratio-controlled transmission. In the latter, since the variator maintains a chosen drive ratio, torque at the driven wheels is related directly to engine torque. Engine speed control is a relatively straightforward matter since, by maintaining a set drive ratio, the transmission provides a direct relationship between engine speed and vehicle speed. In a torque-controlled transmission, in which drive ratio is not the control variable and is permitted to vary, the engine and wheels can be thought of as being effectively de-coupled from one another. Wheel torque is controlled by the variator rather than by engine torque. Engine speed is not constrained to follow vehicle speed. Instead the control signal applied to the variator determines a loading torque applied by the variator to the engine. Combustion within the engine creates an engine torque. The sum of the loading torque and the engine torque acts upon the inertia referred to the engine (contributed by masses in both engine and the transmission) and so determines engine acceleration. While the loading torque and the engine torque are equal and opposite, engine speed is constant. Changes in engine speed result from an inequality between these torques. Dynamic matching of engine torque to loading torque is thus fundamental to management of the drive line as a whole and of engine speed in particular. Failure to manage the balance would allow unwanted changes in engine speed.
Some issues relating to engine speed management are addressed in U.S. Pat. No. 6,497,636 (Schleicher et al) which, so fax as the present applicant has been able to understand the language of this document, concerns itself with transmission and engine adjustments needed to bring the engine to the desired operating point (engine speed and engine torque).
The profile of changes in engine speed is important to the “driveability” of the vehicle. The fact that in a CVT power train the engine is typically run at low speed and high torque (to provide high fuel economy) makes the management of engine speed especially important. When the driver calls for an increase in power the engine, already operating close to its maximum torque, must typically be accelerated in a controlled manner in order to be capable of providing the required power.